I discovered that if you open the shutter in a defocused state, while the fireworks start, then gradually focus on the subject right before closing the shutter, you get a very interesting effect seen on Fig. 1.

Fig. 2 shows the reverse of the effect when starting in a focused state and defocus as the fireworks blosom.

These photos were taken on Jan. 1, 2013.

Fig. 1
Fig. 2